A recombinant double mutant (Hb), E6V/L88A(b) was constructed to study the strength of the primary hydrophobic interaction in the gelation of sickle Hb, i.e., that between the mutant Va-6(b) of one tetramer and the hydrophobic region between Phe-85(b) and Leu-88(b) on an adjacent tetramer. Thus, a construct encoding the donor Val-6(b) of the expressed recombinant HbS and a second mutation encoding an Ala in place of Leu-88(b) was assembled. The doubly mutated b-globin gene was expressed with the normal human a-chain, which is on the same plasmid, to produce a double Hb tetramer. Characterizations of the Hb double mutant by mass spectrometry, by HPLC, and by peptide mapping of tryptic, digests of the mutant b-chain were consistent with the desired mutations. The absorption spectra in the visible and the ultraviolet regions were practically superimposable for the recombinant Hb and the natural Hb purified from human red cells. Circular dichroism studies on the overall structure of the recombinant Hb double mutant and the recombinant single mutant, HbS, showed that both were correctly folded. Functional studies on the recombinant double mutant indicated that it was fully cooperative. However, its gelation concentration was significantly higher than that of either recombinant or natural sickle Hb, indicating that the strength of the interaction in this important donor-acceptor region in sickle Hb was considerably reduced even with such a conservative hydrophobic mutation. The work has been published in Protein Science 3 (1994) 1206. A series of further mutants are currently under study.